The semiconductor industry has had tremendous success in delivering ever more cost effective chips to market through the use of scaling. However, while scaling works well in device or front end of semiconductor processing, device wiring is not amenable to scaling and results in degraded interconnect resistance and/or capacitance. To alleviate this problem, the industry has been migrating to the use of a lower resistance conductor (copper) and is also introducing lower-k insulators to reduce capacitance. Newly developed insulators in the ultra-low-k (ULK) range (k<2.5) are generally characterized by a great deal of porosity (e.g., 30-50%.) These materials are extremely difficult to integrate since they are susceptible to contamination from other wet chemicals and gases.
Conventional dual damascene ULK integration schemes rely on a series of higher-k hard-mask films to protect the surface of the porous and fragile ULK films. The two most widely used process sequences are the “via first” scheme and the conventional “dual hard-mask”.
FIGS. 2A-2F show schematics illustrating in-process structures (identified as 300A-300F) resulting after each step in a conventional “via first” dual damascene ULK integration process. The “via first” process begins with deposition of a sequence of insulator films over the previous metal wiring level (not shown in the figures.) As shown in FIG. 2A, this insulator stack 300A is comprised of etch stop film 302, ULK dielectric layer 304, first hard-mask layer 306, second hard-mask layer 308, antireflective coating 310, and photoresist layer 312. The etch stop film 302 must have a reasonably low-k value and the ULK dielectric layer 304 must have a high etch rate ratio with respect to the etch stop film 302. If the prior (underlying) metal wiring layer is copper, then this etch stop film 302 must also be a Cu diffusion barrier. The most commonly used etch stop films for this application are Si3N4 and SiCN.
The first hard-mask layer 306 is intended to remain as part of the dual damascene structure after processing to protect the fragile, porous ULK dielectric layer 304 from the effects of a subsequent chemical mechanical polishing (CMP) step. The second hard-mask layer 308 is a material that provides oxidation resistance and enables photo rework through ash removal of the antireflective coating 310 and the photoresist layer 312. Typical materials used for first hard-mask layer 306 are SiCOH or SiC while SiO2 and Si3N4 are typically used for second hard-mask layer 308. The second hard-mask layer 308 is a sacrificial layer and is removed during subsequent CMP of the Cu and barrier materials.
As reflected by structure 300B of FIG. 2B, the photoresist layer 312 is then imaged in the desired via pattern and then reactive ion etching (RIE) is used to transfer the pattern down through the antireflective coating 310, hard-mask layers 306 and 308 and through the ULK dielectric layer 304 while stopping on the SiCN etch stop, thus forming via 324. As depicted in FIG. 2C, any remaining photoresist layer 312 and antireflective coating 310 are removed by plasma ashing with mixtures of oxygen and other gases to obtain structure 300C. Another prior art alternative is plasma ashing of the resist followed by solvent removal of the antireflective coating layers. RIE polymer on the via sidewalls 314 is traditionally removed by wet cleans. It has been shown that both the oxygen plasma ashing and wet clean processes, which were traditionally used with SiO2 dielectrics, can damage the porous ULK dielectric layer 304 materials and so are not optimal because they are detrimental to the process and can reduce the reliability and performance of the circuits thus formed.
As shown in FIG. 2D, antireflective coating and photoresist materials are next re-applied to the patterned wafer to obtain structure 300D. A bottom antireflective coating 316, which will fill the etched vias (via 324, for example) and planarize the surface is applied first. A third hard-mask layer 318 is then applied, which will allow photo rework, and this is followed by a standard antireflective coating 320 and photoresist layer 322. One prior art alternative to this scheme is to completely remove and then re-apply all anti-reflective coating and photoresist layers during each photo rework operation.
FIG. 2E shows the stack (structure 300E) after imaging the photoresist layer 322 in a desired trench pattern and after transferring through RIE the pattern down through the third hard-mask layer 318, bottom antireflective coating 316, second hard-mask layer 308, first hard-mask layer 306 and partially into the ULK dielectric layer 304, thus forming a trench 326. It should be noted that the bottom antireflective coating 316 material should have a slightly faster RIE etch rate than the ULK dielectric layer 304 material or un-etched protrusions “fences” will remain around the via periphery.
Now referring to FIG. 2F, RIE is used to remove any remaining photoresist layer 322, antireflective coating 320, third hard-mask layer 318, bottom antireflective layer 316, and to open the underlying etch stop film 302, thus completing via 324 and obtaining structure 300F. It should again be noted that oxygen RIE processes and any needed wet cleans have been shown to permeate and degrade the porous ULK dielectric layer 304 material, to the detriment of the process and to the reduced reliability and performance of the circuits thus formed.
FIGS. 3A-3G show schematics illustrating in-process structures 400A-400G corresponding to stages in the prior art conventional “dual hard-mask” dual damascene ULK integration scheme, a second widely used formation process.
As shown in FIG. 3A, the “dual hard-mask” process begins with deposition of a sequence of insulator films over the previous metal wiring level. Similar to the “via first” scheme discussed above, this “dual hard-mask” insulator stack (structure 400A) is also comprised of an etch stop film 402, a ULK dielectric layer 404, a first hard-mask layer 406, second hard-mask layer 408, an antireflective coating 410, and a photoresist layer 412. The first hard-mask layer 406 in this integration scheme is intended to remain as part of the dual damascene structure after processing to protect the fragile, porous ULK dielectric layer 404 from the effects of subsequent CMP. A RIE etching chemistry for which the second hard-mask layer 408 has a high etch rate with respect to that for the first hard-mask layer 406 must be used initially and subsequently other chemistries must be applied for which the second hard-mask layer 408 has a low etch rate with respect to the ULK dielectric layer 404. The second hard-mask layer 408 is a sacrificial layer and is removed during CMP of the Cu and barrier materials.
As shown in FIG. 3B, the photoresist layer 412 is then imaged in the desired trench pattern and RIE is used to transfer the pattern down through the antireflective coating 410 and second hard-mask layer 408, stopping at first hard-mask layer 406, thus forming trench pattern 414 in the second hard-mask layer 408 in structure 400B.
As reflected in structure 400C depicted in FIG. 3C, the remaining photoresist layer 412 and antireflective coating 410 are either removed by plasma ashing with mixtures of oxygen and other gases or alternatively by using wet solvents.
As shown in FIG. 3D, antireflective coating 416 and photoresist layer 418 are next re-applied to the patterned wafer to obtain structure 400D.
FIG. 3E shows resultant structure 400E following imaging of the photoresist layer 418 in the desired via pattern followed by RIE transfer of the pattern down through the antireflective coating 416, second hard-mask layer 408, first hard-mask layer 406, and partially into the ULK dielectric layer 404, forming partial via 420. It should be noted that the antireflective coating 416 material should have a similar etch rate to the second hard-mask layer 408 during this process.
As shown in FIG. 3F, an ashing or RIE process is then used to remove any remaining photoresist layer 418 and antireflective coating 416 from the workpiece to obtain structure 400F.
The structure 400G illustrated in FIG. 3G shows that next a RIE process is used to extend the second hard-mask layer's trench pattern 414 through first hard-mask layer 406 and into the ULK dielectric layer 404, forming trench 424, while simultaneously completing the via etch and thus opening up the underlying etch stop film at the bottom of the completed via 422. The second hard-mask layer 408 is partially consumed during this process and is intended to be completely removed during the subsequent barrier and Cu CMP process. It should also be noted that since a portion of the via 422 is not protected by resist or a hard-mask during this RIE process, the via shape tends to become elongated and to develop a sloped region 426 as also shown in FIG. 3G. A subsequent argon pre-clean process for the Cu barrier and seed layer tends to sputter material from this sloped region 426 into the bottom of the via 422, which is typically a copper metal layer, and causes contamination and reliability problems.
A problem common to both of these prior art integration schemes (“via first” and “dual hard-mask” is that RIE is subject to micro-loading effects that lead to poor control of trench depth and shape and, therefore, poor control of the wiring resistance and capacitance. The inability to definitively specify wiring resistance and capacitance to the design community has a negative impact on chip performance.
Since RIE leaves the etched ULK dielectric surfaces as an open porous structure, these interfaces are not compatible with the desire to use ALD or CVD techniques for the subsequent barrier and seed layer processes.
In addition, both of these prior art integration schemes utilize multiple hard-masks for processing which are complex and costly. The final insulator dual damascene structure also retains a hard-mask layer which raises the effective k value of the insulator structure and also serves as a focal point for leakage, delamination, and other potential reliability problems.
There are a number of problems shared by both of these integration schemes. First, the resulting final structures produced by these processes retain one or more of the hard-mask layers. This raises the effective k value of the insulator structure and is not desirable. There is additional motivation to minimize the use of these hard-mask layers in that every additional material interface is a potential source for electrical leakage, delamination, or other reliability problems. Also, the etched surfaces of the trench and via structure are open to contamination from subsequent process steps. For example conventional wet or dry stripping processes have been shown to contaminate the ULK films. In addition, it is desirable to use chemical vapor deposition (CVD) or atomic layer deposition (ALD) processes to deposit the barrier film in subsequent processing steps. It has been shown that ALD and CVD processes penetrate the porous RIE etched sidewalls and raise the k value of the ULK films. Finally, conventional RIE is subject to seasoning and micro-loading effects that lead to a great deal of variability in the shape and depth of the trench and, therefore, the wiring resistance and capacitance. This inability to definitively specify wiring resistance and capacitance to the semiconductor design community has a negative impact on chip performance.